Fuel cells are devices that directly convert chemical energy of reactants, i.e., fuel and oxidant, into direct current (DC) electricity. For an increasing number of applications, fuel cells are more efficient than conventional power generation, such as combustion of fossil fuel, as well as portable power storage, such as lithium-ion batteries.
In general, fuel cell technology includes a variety of different fuel cells, such as alkali fuel cells, polymer electrolyte fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells and enzyme fuel cells. Today's more important fuel cells can be divided into several general categories, namely (i) fuel cells utilizing compressed hydrogen (H2) as fuel; (ii) proton exchange membrane or polymer electrolyte membrane (PEM) fuel cells that use alcohols, e.g., methanol (CH3OH), metal hydrides, e.g., sodium borohydride (NaBH4), hydrocarbons, or other fuels reformed into hydrogen fuel; (iii) PEM fuel cells that can consume non-hydrogen fuel directly or direct oxidation fuel cells; and (iv) solid oxide fuel cells (SOFC) that directly convert hydrocarbon fuels to electricity at high temperature.
The chemical reactions that produce electricity are different for each type of fuel cell. For DMFC, the chemical-electrical reaction at each electrode and the overall reaction for a direct methanol fuel cell are described as follows:CH3OH+H2O→CO2+6H++6e−  Half-reaction at the anode:1.502+6H++6e−→3H2O  Half-reaction at the cathode:CH3OH+1.5O2→CO2+2H2O  The overall fuel cell reaction:
Due to the migration of the hydrogen ions (H+) through the PEM from the anode to the cathode and due to the inability of the free electrons (e−) to pass through the PEM, the electrons flow through an external circuit, thereby producing an electrical current through the external circuit. The external circuit may be used to power many useful consumer electronic devices, such as mobile or cell phones, calculators, personal digital assistants, laptop computers, and power tools, among others.
DMFC is discussed in U.S. Pat. Nos. 4,390,603 and 4,828,941, which are incorporated by reference herein in their entireties. Generally, the PEM is made from a polymer, such as Nafion® available from DuPont, which is a perfluorinated sulfonic acid polymer having a thickness in the range of about 0.05 mm to about 0.50 mm, or other suitable membranes. The anode is typically made from a Teflonized carbon paper support, which is a carbon paper coated on one side with polytetrafluoroethylene (PTFE) (TEFLON® is a registered trademark of the E.I. DU PONT DE NEMOURS AND COMPANY Corporation) with a thin layer of catalyst, such as platinum-ruthenium, deposited thereon. The cathode is typically a gas diffusion electrode in which platinum particles are bonded to one side of the membrane.
In a chemical metal hydride fuel cell, sodium borohydride is reformed and reacts as follows:NaBH4+2H2O→(heat and/or catalyst)→4(H2)+(NaBO2)H2→2H++2e−  Half-reaction at the anode:2(2H++2e)+O2→2H2O  Half-reaction at the cathode:
Suitable catalysts for this reaction include platinum and ruthenium, and other metals. The hydrogen fuel produced from reforming sodium borohydride is reacted in the fuel cell with an oxidant, such as O2, to create electricity (or a flow of electrons) and water by-product. Sodium borate (NaBO2) by-product is also produced by the reforming process. A sodium borohydride fuel cell is discussed in U.S. Pat. No. 4,261,956, which is incorporated by reference herein in its entirety.
One of the more important features for fuel cell application is transportation of a liquid fuel from the fuel storage area to either the fuel cell, such as transporting methanol to a DMFC, or a liquid fuel reactant to a reaction chamber, such as transporting water and additives to react with a metal hydride. Known methods of transporting liquid fuel/reactant include wicking or capillary action, pressurizing the liquid fuel/reactant. Among the challenges encountered with these methods include controlling the flow rate with wicking fuel and maintaining a steady pressure on the fuel with pressurized source.
Hence, there remains a need in the art for improved methods of transporting liquid fuel/reactant.